In the lighting field, LED light sources have been widely used, attributed to their advantages such as high luminous efficiency, long lifetime, low cost, etc. Fluorescent lamps, as traditional light sources, are gradually replaced by LED light sources. In the process of replacing a fluorescent lamp with a LED light source, a driving circuit of the LED light source is always required to obtain power from an electronic ballast, such that the LED light source can be compatible with the electronic ballast of the fluorescent lamp, and the old fluorescent lamp can be easily replaced and updated by a LED light source.
A working principle of the electronic ballast is described as follows. The electronic ballast generates a voltage having a peak higher than 500V to breakdown the fluorescent lamp in the early start, and establishes a discharge channel therein. Then, the electronic ballast maintains an arc in the discharge channel with a constant current. Further, when the fluorescent lamp is disconnected and reconnected, the electronic ballast can generate a high voltage to breakdown the fluorescent lamp once again. If a lamp cap of the fluorescent lamp is not properly installed in a lamp holder, or poor contact between the lamp cap and the lamp holder occurs in a working process, the electronic ballast may output a high voltage continuously. Because the lamp cap and the lamp holder are not well contacted, there is a large contact resistance between them. The high voltage may generate an arc between the lamp cap and the lamp holder, which is known as an arcing phenomenon. As a result, a high temperature is generated; the lamp cap and the lamp holder are deformed; and finally the fluorescent lamp may fall off from the lamp holder. In a serious case, surrounding combustible materials may be ignited and a fire may be caused. In the replacement of the fluorescent lamp with a LED light source, only the lamp is replaced, but the old lamp holder is stilled used. The old lamp holder usually has problems such as aging, wearing and surface oxidation of metal pieces. Thus, compared with a new lamp holder, the old lamp holder is more prone to having a problem of poor contact. As the LED light source usually has a plastic lamp cap, the high temperature generated by the arcing may easily deform the plastic lamp cap, resulting fall off of the LED light source and even a fire. Therefore, there is a need for protecting the LED light source from the arcing.
In the conventional technology, some electronic ballasts may have a protection circuit for preventing the arcing. The protection circuit detects a changing rate of an output current or an output power, so as to determine whether an arcing phenomenon occurs. The protection circuit can cut off the output of the electronic ballast or decrease the output current, so as to eliminate or suppress the arcing. When working with the fluorescent lamp, the protection circuit can eliminate and suppress the arcing effectively. However, when the fluorescent lamp is replaced by a LED light source, as the LED light source has non-linear devices such as diodes, switch devices, etc., current distortion may occurs, and the protection circuit working in a current changing rate detection manner, may be falsely triggered. Moreover, the LED light source replacing the fluorescent lamp has a lower power, which may greatly reduce protection effect of the protection circuit working in a power detection manner. Hence, in the conventional technology, a thermal fuse is added at a pin of the lamp cap. When an arcing occurs and a high temperature is generated, the thermal fuse is melted, such that the LED light source is disconnected from the current loop. However, after the thermal fuse is melted, the LED light source is damaged and is unable to revert.